This invention pertains to spectrally controlled illumination.
In color photography a pleasing and life-like end result is invariably desired. The exigencies of the original photographing, the imperfections of the photographic medium, and variations in photographic processing frequently result in a color negative that is less than perfect. Suitable variation of the spectral content of the printing illumination is a significant factor in improving the color positive made from a defective color negative.
While this has been known, and efforts have been made to mitigate imperfections in the negatives, unobvious imperfections in printer performance have remained.
The prior art has proposed, of course, employing essential printer elements, as a plurality of spectrally distinct sources of illumination emitting red, green and blue light, a diffusing surface, the negative, and positive paper, with suitable optics.
Spectrally distinct photocells have also been proposed to obtain a measure of the total spectrum of the printing illumination by connecting these photocells in pairs and to a null-indicating meter for manual color correction of the illumination. However, a necessary teaching as to the input of illumination to the photocells and its relation to the printing illumination has been absent.
A similar proposal has been made in which exposure intervals for each individual primary color are terminated mechano-optically according to data from color analysis of the negative through the intermediary of a computer.
Two further similar proposals have been made to use a cathode-ray tube for flying-spot illumination of the color negative to obtain colormetric data on the negative. A computer interprets this to prevent "subject failure" and to otherwise obtain a positive print of the best possible color fidelity by the subtractive process of color photography.
A lamphouse for premixing colored light from filtered lamp sources has been proposed that employs an inwardly recessed roof, with the individual source light being introduced thereto through rod screens. Means to sample the integrated light were not provided.
Another lamphouse substitutionally provided for a black and white enlarger employs 6 lamps, each having a color filter and shining upon a house-roof shaped mixing chamber, which chamber has only diffusely reflecting white walls. Color balance is manually obtained by rheostats.
Another colored light mixing structure employs relatively long and narrow individual light tunnels that are lined throughout with specularly reflecting surfaces; i.e., mirrors.